Single phase alternating current electric motors conventionally are provided with two windings on a stator core, inductively coupled to the rotor of the motor. Such motors are widely used for a variety of different purposes and range in size from very small fractional horsepower motors on up to multiple horsepower sizes. Single phase motors are particularly popular since most home and business alternating current supplies are in the form of single phase power supplies.
The two windings of single phase electric motors comprise a start winding and a run winding which are connected to the source of operating power. These two stator windings surround and are inductively coupled to the rotor which rotates a shaft to produce the motor output. Rotors are made in a number of different configurations, such as squirrel cage rotors, high resistance rotors, low resistance rotors, wound rotors or multiple winding high and low resistance rotors. All of these configurations, along with various stator winding arrangements are well known in the electric motor industry.
Typically, the start winding is made of relatively small diameter wire and the run winding is made of relatively large diameter wire compared to the diameter of the start winding. These windings are physically and electrically, angularly displaced from one another on the stator.
In conventional capacitor-start and capacitor-start/capacitor-run motors, a starting capacitor is connected in series with the starting winding and a switch. At motor start-up, the switch is closed and the capacitor, in conjunction with the relatively small diameter start winding, produces a leading current in the starting winding which is approximately equal to and approximately 90.degree. displaced in phase from the lagging current in the main or run winding of the motor. Such arrangements produce high values of starting torque.
Usually the switch in a conventional capacitor-start motor is a centrifugal or thermal switch or it is a current operated switch connected in series with the capacitor and start winding across the input terminals. The run winding then is connected in parallel with this series-connected starting circuit. In such capacitor start motors, the starting condition is such that the instantaneous locked rotor current is high and the motor starting current demand factor also is high. As a consequence, such motors undergo relatively high operating temperatures and require some type of switch for disconnecting or opening the start winding circuit after a pre-established rotational speed of the motor is reached. Because the start winding of such motors generally is a relatively small diameter wire, overheating can and frequently does occur. Such overheating can result in a relatively limited life of the start winding due to burnout. This is the reason for the switch to disconnect the start winding from the motor circuitry after some pre-established operating condition has been reached.
Capacitor run motors also are utilized to produce relatively high starting torque, but instead of opening the circuit or switching out the start winding and capacitor during the operation of the motor, the start winding and capacitor remain in circuit throughout the operation. The parameters of the capacitor and start winding, however, are selected such that the primary current through the motor during its normal operation takes place through the run winding, with only a smaller residual current flowing through the start winding and the capacitor.
A third type of motor incorporates both capacitor start and capacitor run features with a starting capacitor being switched out of the circuit after the start-up conditions are met and a parallel run capacitor remains in the circuit, so that after start-up, this motor operates in the same manner as a conventional capacitor-run motor.
Motors of all of the above types also may employ the series-resonant capacitor configuration of my co-pending application, Ser. No. 07/144,544.
Conventional capacitor motors of the different types discussed above, and the motors of my co-pending application, Ser. No. 07/144,544, all are subject to an inherent operating disadvantage which takes place when the motor is turned off or switched to its "off" position. When the power supply is disconnected from the motor windings, such motors, particularly capacitor-run or capacitor-start/capacitor-run motors, are quite noisy as a result of capacitor discharge taking place through one or both of the windings immediately following the opening of the power switch to the motor. This noise is due to vibration or chattering of the rotor caused by the somewhat erratic discharge current through the operating coils of the motor. In addition, arcing across switch contacts also takes place and can be heard. This chattering and arcing is detrimental to the life of the motors in addition to being annoying. The condition is particularly noticeable for large horsepower motors which have relatively large capacitors in the motor circuits.
It is desirable to provide a motor which has the advantages of prior art capacitor motors, but which does not have the disadvantages of noise and vibration when the motor is turned off.